1. Field of the Invention
This invention relates to the communication field, and more particularly to a phase-locked transceiver system.
2. Description of Prior Art
Basically, receiver-transmitter or transceiver units available in the past employ either an analog mixer or a frequency synthesizer approach, the latter approach sometimes being referred to as a phase-locked loop scheme. Illustrative of the analog approach, a radio frequency signal is combined with an intermediate frequency signal in an analog mixer. However, the analog mixer can often introduce deleterious noise excursions into the system thus limiting the overall quality of transmission or reception.
A primary reason for employing phase-locked loop systems or frequency synthesizers is to minimize the noise spurious response problems associated with the analog approach. The phase locked loop system essentially employs a reference oscillator, digital detector, active filter network, and a voltage controlled oscillator. A digital counter is connected in feedback relation between the voltage controlled oscillator and the digital phase detector. The digital phase detector compares the reference oscillator signal with the output from the digital counter for producing an error voltage which is applied to the VCO. The VCO generates a signal which is fed to the counter in order to generate a digital correction signal for the digital phase detector. Ideally, the system will reach a steady state wherein the two signals applied to the digital phase detector, that is the reference signal and the signal from the feedback counter are of identical phase and frequency. In some known prior art approaches of this type, both the transmit and receive sections of the system required their independent phase-locked loop system. Accordingly, a transceiver required two phase-locked loops or used an analog mixing approach.
It has been suggested to employ a single phase-locked loop system for a transceiver. However, known approaches using a single phase-locked loop system still suffer a major drawback. When it is desired to generate different transmit and receive frequency signals, it is necessary first to manually set the desired receive frequency into the system and during transmit resetting to the desired transmit frequency. For example, in a duplex or repeater mode of operation with a transceiver it is desirable to receive at a first predetermined frequency and to transmit at a second predetermined frequency where the receive frequency is offset from the transmit frequency a predetermined amount. Prior Art approaches require a person to manually select the transmit frequency in order to generate that desired frequency, and thereafter manually select or dial another setting in order to obtain the receive frequency.